Extended cascode amplifier

ABSTRACT

Disclosed is a cascode amplifier circuit with improved current gain, linearity, overload performance and bandwidth which uses a step-down (of voltage) transformer between a driver stage and an output stage. One end of the primary of the transformer is connected to a driver transistor&#39;s collector and the other end is connected to the collector of a grounded-base output transistor. The transformer secondary is connected across the base and emitter of the output transistor. The transformer windings are phased such that a large driver current is forced through a high impedance, i.e. the primary winding impedance which is a multiple of the common base input impedance of the output transistor, resulting in power gain. The amplifier is capable of high maximum overloaded power output free of &#34;break-up&#34; of the waveform compared with conventional cascode operation, as well as greater power gain. By using transistors with high resistivity base material the amplifier operation is resistant to degradation by ionizing radiation and protected against secondary breakdown.

BACKGROUND OF THE INVENTION

This invention relates to cascode amplifiers, and in particular what istermed herein an extended cascode amplifier.

Some early types (Circa 1961) of silicon transistors, e.g. GeneralElectric type 4JX 10B513 NPN, exhibited, in common emitterconfiguration, unacceptably high saturation voltage (6 V at 0.5 A Ic).This was due to the use of base semiconductor material having aresistivity that was chosen to be much too high in aid of the primaryobjective of making transistors that would be much more thermally stablethan previously used germanium transistors in contemporary (circa 1961)design circuits (common emitter).

The use of the common base configuration was the only obvious expedientto overcome the saturation voltage. In this mode of operation the swingof collector voltage along a power load line is able to reach all theway down to zero volts V_(c) in contrast to saturation near five or sixvolts in the common emitter configuration. However, common baseoperation involves very low input impedance to the common base stage aswell as an absence of current gain in the output stage.

The present invention enables the current gain to be augmented in thedriver stage of a cascode by a step-down transformer, while preservingessentially the voltages at the input collector, the input stage baseand the output collector substantially the same magnitudes they wouldhave in a conventional cascode circuit, i.e. one having an interstagetransformer with a turns ratio not substantially different fromone-to-one.

SUMMARY OF THE INVENTION

An object of the invention is to provide a cascode type of amplifierhaving improved current gain while essentially preserving the voltagemagnitudes of a conventional cascode amplifier circuit.

Another object of the invention is to provide a transistor amplifiercapable of reasonably efficient operation in the presence of ionizingradiation. Briefly, the first object is achieved by providing a cascodeamplifier circuit having a step-down (of voltage) transformer between adriver stage and an output stage. One end of the primary of thetransformer is connected to an input (driver) transistor's collector andthe other end is connected to the collector of a grounded-base outputtransistor. The transformer secondary is connected across the base andemitter of the output transistor. The transformer windings are phasedsuch that a large driver current is forced through a high impedance,i.e. the primarywinding impedance which is a multiple of the common baseinput impedance of the output transistor, resulting in power gainaugmentation (over all). The amplifier is capable of high maximum poweroutput free of "break-up" of the waveform compared with conventionalcascode operation, as well as greater power gain, while retaining"ballasting" in the power transition(s).

The second object may also be achieved by using transistors with highresistivity base material [i.e. ballasting]. [Ref. I.E.E.E. SocietyCircuits & Systems 1978 Symposium Record Transient Radiation Effects onElectronics of Aircraft Systems, T. J. Lambarski, pp. 119 to 123.] [Ref.R. J. Widlaz I.S.S.C.C. 1981 Paper WAM 3.5.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified schematic of a conventional cascode amplifier,

FIG. 2 comprises common emitter collector characteristic curves of atransistor having high base resistance,

FIG. 3 comprises common base collector characteristics of a transistorhaving high base resistance (i.e. ballasting),

FIG. 4 is a schematic of an extended cascode amplifier circuit accordingto the invention, and

FIG. 5 is a schematic of a modified circuit according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is presented merely to illustrate the basic configuration of aconventional cascode circuit. A signal at the input 10 feeds agrounded-emitter transistor 11 which, in turn, feeds a grounded-basetransistor 12. The output 14 is taken across a load 13 connected betweenground and the collector of output transistor 12. However, with thiscircuit configuration, the input impedance to transistor 12 is very lowRef. Principles of Transistor Circuits, Ed. by R. F. Shea Wiley, 1953,p. 207, etc. (increasing with freq.) and there is no current gain in theoutput stage.

Early types of silicon transistors, such as General Electric type 4JX10B513 NPN, were formed using base semi-conductor conductor material ofhigh resistivity in aid of the (then) primary objective of makingtransistors that would be much more thermally stable (in contemporarystate of the art circuits) than the hitherto used germanium transistors.However, as can be seen from FIG. 2, the common emitter collectorcharacteristic curves for such transistors exhibited unacceptably highsaturation voltage (6.0 V. at 0.5A.Ic) for use in common emitter mode.On the other hand, transistors with high resistivity base material areresistant to damage by ionizing radiation and it would be useful to usethem for circuits requiring radiation hardening, e.g. in military andspace applications.

The use of the common base configuration was the only obvious expedientto overcome the saturation voltage problem discussed above. This isshown in FIG. 3 showing common base collector characteristics of the 4JX10B 513 NPN. In this mode of operation the swing of collector voltagealong a power load line is able to reach all the way down to zero voltsV_(c) in contrast to saturation near five or six volts with the commonemitter mode. However, a disadvantage of common base operation is thatit involves very low input impedance to the common base stage.Furthermore, there is no current gain in the output (usually poweroutput stage).

The present invention enables the current gain to be augmented in thedriver stage of a cascode by a step-down transformer, reducing thecurrent and voltage levels in the driver by an order of magnitude whilepreserving essentially the voltages at the input collector, the inputstage base and the output collector in the same magnitudes that theywould have had in a cascode circuit. The invention also allows the useof high base resistivity transistors in the output resulting in aradiation hardened circuit and immunity to secondary breakdown.

Referring to FIG. 4, an extended cascode amplifier circuit according tothe invention is shown wherein similar reference numerals are used forsimilar components as in FIG. 1. In this arrangement, the collector ofdriver transistor 11 is connected through the primary winding 15 of atransformer 16 to the collector of output transistor 12. The secondarywinding 17 of transformer 16 is connected between the base and emitterof transistor 12. The load 13 is coupled to the collector of transistor12 via a transformer 20 with B" for the transistors supplied through theprimary 21 of transformer 20. A capacitor 22 isolates the B from groundwhile connecting AC through the primary 21 to ground.

The windings 15 and 17 of transformer 16 are phased in the manner shownso that a voltage rise at the collector of transistor 12 corresponds toa reduction of instantaneous signal current in the primary winding 15 oftransformer 16. This results in instantaneous potentials with respect toground at the collector of input transistor 11 equal in magnitude to thedifference in the instantaneous magnitudes of the signal voltage acrossthe primary winding and the output voltage. Another way of expressingthis is to say that a large driver current is forced through a highimpedance, namely the impedance of the primary winding 15 which is, inmagnitude, a multiple of the common base input impedance of the outputtransistor 12 because of the turns ratio of transformer 16. The voltagedrop in coil 15 is almost exactly compensated by the voltage rise incoil 21 (at choice of designer) so that the differences in thesevoltages, which is the voltage of the collector-emitter loop oftransistor 11, can be chosen as desired, e.g. very small. By this means,a power gain is produced as compared to the power gain obtainable withthe same transistors connected in conventional cascode fashion. Althoughthe forcing of a large current through a high impedance (load impedance)is also a mechanism that is present in the conventional cascode, thedriver has to supply, in that case, a current equal in magnitude to theoutput transistor emitter current.

The modification of cascode operating current in the input transistorthat results from this "extended cascode" arrangement creates, aroundthe output transistor common base stage, a positive voltage feedbackproportional to the ratio: ##EQU1##

With input transistors having the commonly encountered pentode-likeconstant current static characteristic curves (common emitter), thedenominator of the ratio is much larger than the numerator, so that oneachieves what may be termed microscopic feedback with macroscopiceffect, the principle "macroscopic" effect being a substantial increasein maximum overloaded output free of "break-up" of the waveform comparedwith conventional cascode operation, as well as greater power gain andbetter linearity below overload.

In known circuits the driver normally loses control when the inputsignal is at a high level and cuts off before the output transistorreaches maximum output. In other words, the driver ceases to be a driveron overload. In the present invention, the driver never loses control.The load line is at the choice of the circuit designer by changing theturn ratio of the interstage transformer 16. A positive load line shouldbe selected. The driver transistor only makes small excursions even whenthe output transistor is clipping so that the maximum clipped output ismuch larger than with normal amplifiers. This is very useful for P.A.(Public Address) systems for good articulation of speech in high ambientnoise conditions, e.g. [Designing for other than min. driver voltages,permits substantial driver current to be added to the output current.]

The transformer 16 is, for high frequency or wide-band, a transmissionline transformer, these being capable of wide bandwidth operation. Thesetransformers, known per se, consist of transmission line (twisted pair,coaxial, or strip transmission line) wound on a high-permeability coresuch as ferrite or permalloy. (Ref. G. Guanella, Brown Bovert Review,Vol. 31, 1944, p. 327.)

A modification of the amplifier according to the invention is shown inFIG. 5 in which the input transistor 11 is a field effect transistor andthe output is push-pull transistor 25 having its emitter connected to atap 26 on the primary winding 15 of transformer 16.

In FIG. 5 the transformer 16 may have, for example, a turns ratio of10:1.

What is claimed is:
 1. An extended cascode amplifier circuit comprisinga driver transistor having a base, emitter and collector, the emitterbeing connected to ground, the base being connectable to a signal sourceand the collector being connected to one end of a primary winding of atransformer, the other end of the primary winding being connected to thecollector of an output transistor, said output transistor having a baseand an emitter, the base being connected to ground and, through asecondary winding of the transformer, to the emitter of said outputtransistor, the transformer windings being phased such that a largedriver current is forced through a high impedance of the primary windingwhich is a multiple of the common base input impedance of the drivertransistor.
 2. An amplifier as claimed in claim 1, wherein saidtransistors have sufficiently high base resistance such that saidtransistors' saturation voltage is of the same order of magnitude as asupply voltage used for powering said amplifier circuit.